ARM: let CPUs not being able to run in ARM mode enter in THUMB mode

Some ARM cores are not capable to run in ARM mode (e.g. Cortex-M3). So
obviously these cannot enter the kernel in ARM mode. Make an exception
for them and let them enter in THUMB mode.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Message-Id: 1358162123-30113-1-git-send-email-u.kleine-koenig@pengutronix.de
Acked-by: Nicolas Pitre <nico@linaro.org>

3 files changed, 17 insertions, 3 deletions

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 67874b82a4ed..e04c7793f47e 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1656,8 +1656,9 @@ config HZ
         default 100
 
 config THUMB2_KERNEL
-        bool "Compile the kernel in Thumb-2 mode"
+        bool "Compile the kernel in Thumb-2 mode" if !CPU_THUMBONLY
         depends on CPU_V7 && !CPU_V6 && !CPU_V6K
+        default y if CPU_THUMBONLY
         select AEABI
         select ARM_ASM_UNIFIED
         select ARM_UNWIND
diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S
index 2c228a07e58c..6a2e09c952c7 100644
--- a/arch/arm/kernel/head-nommu.S
+++ b/arch/arm/kernel/head-nommu.S
@@ -32,15 +32,21 @@
  * numbers for r1.
  *
  */
-        .arm
 
         __HEAD
+
+#ifdef CONFIG_CPU_THUMBONLY
+        .thumb
+ENTRY(stext)
+#else
+        .arm
 ENTRY(stext)
 
  THUMB( adr     r9, BSYM(1f)    )       @ Kernel is always entered in ARM.
  THUMB( bx      r9              )       @ If this is a Thumb-2 kernel,
  THUMB( .thumb                  )       @ switch to Thumb now.
  THUMB(1:                       )
+#endif
 
         setmode PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode
                                                 @ and irqs disabled
diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index 3fd629d5a513..8defd638c79e 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -397,6 +397,13 @@ config CPU_V7
         select CPU_PABRT_V7
         select CPU_TLB_V7 if MMU
 
+config CPU_THUMBONLY
+        bool
+        # There are no CPUs available with MMU that don't implement an ARM ISA:
+        depends on !MMU
+        help
+          Select this if your CPU doesn't support the 32 bit ARM instructions.
+
 # Figure out what processor architecture version we should be using.
 # This defines the compiler instruction set which depends on the machine type.
 config CPU_32v3
@@ -608,7 +615,7 @@ config ARCH_DMA_ADDR_T_64BIT
         bool
 
 config ARM_THUMB
-        bool "Support Thumb user binaries"
+        bool "Support Thumb user binaries" if !CPU_THUMBONLY
         depends on CPU_ARM720T || CPU_ARM740T || CPU_ARM920T || CPU_ARM922T || CPU_ARM925T || CPU_ARM926T || CPU_ARM940T || CPU_ARM946E || CPU_ARM1020 || CPU_ARM1020E || CPU_ARM1022 || CPU_ARM1026 || CPU_XSCALE || CPU_XSC3 || CPU_MOHAWK || CPU_V6 || CPU_V6K || CPU_V7 || CPU_FEROCEON
         default y
         help